Hosts with gazillions of subdomains (user-name.example.com) can implement scripts to generate XML sitemaps for each user on their particular sub-domain (user-name.example.com/sitemap.xml). They can update those sitemaps when a user uploads/creates or changes content.

Because Google accepts sitemaps on a per server base only, it's not possible to create a sitemap index file containing all user-sitemaps on the domain's root level (example.com or www.example.com). That is, the submission of huge amounts of subdomain-sitemaps can't be done via the offial route (manually using a Google account).

A suitable solution for mass submissions of subdomain-sitemaps is anonymous pinging, because it can run fully automated.

That's what I do for my own hosting clients. When they add new content, Google gets pinged with the URL of the sitemap, which is generated on the fly via either a direct file the user maintains or via the 404 handler (which executes a script that generates a real-time sitemap and sets the status code to 200).

It works perfectly since I don't need stats for them. However, if it were necessary to debug a problem, it is possible to add individual sites to my sitemaps account as well. Also, all clients can verify their sitemaps to view their stats and track down crawling problems themselves.